Lets say you happen to be a startup that produces an app for Salesforce or a plugin for Wordpress. Wouldn't it be great to have a big list of every company that uses either one of those tools?
Option: 1 - Go buy a list from some list vendor or biz dev company. Sure, it's an option but if money is tight check out option #2.
Option: 2 - Go to a job board aggregator like Indeed or Monster.com. Pick a location and enter Salesforce.com as a keyword. BOOM! You just got a list of companies that use Salesforce.com because it is a required skill in the job description.